Field of the Invention
The invention relates to emissions from long-lived luminescent compounds, and more particularly, to methods and systems for fine-tuning the emission yields and glow time of such compounds by coupling to surface plasmons of metallic particles.
Related Art
Fluorescence, phosphorescence and related technologies (herein referred to as “luminescence” to include in this context all processes where energy is emitted subsequent to an excitation process triggered by absorption of electromagnetic radiation) are now widely used in a variety of analytical schemes. Luminescent materials are used as tracers on the basis of the high detection sensitivity that can be achieved, but are also used as environmentally responsive “probes” to monitor local conditions, such as pH, ion concentrations, oxygen tension etc. Luminescent compounds can also be used to detect and sometimes quantify the proximity of an agent which is able to modify the emission process on close approach or contact.
In recent years, there has been growing interest in understanding the interactions of fluorophores with metallic particles [1, 2]. Spectral properties of fluorophores are found to be dramatically changed near particulate surfaces due to surface plasmon interactions. A huge enhancement in fluorescence intensity along with an increased absorption, a decrease in decay times and an increase in photo-stabilities and angular-dependent emission are also frequently observed [3, 4]. The phenomenon subsequently been named as metal-enhanced fluorescence (MEF) by Geddes [3]. The current interpretation of MEF has been underpinned by a radiating plasmon model (RPM) [5], whereby non-radiative energy transfer occurs from excited fluorophores to surface plasmons in non-continuous films due to fluorophore-plasmon near field coupling. The surface plasmons efficiently radiate the emission of the coupling fluorophores. Studies have been conducted to show increased detectability [6] and photostability of fluorophores [7], chemiluminescent compounds [8] and improved DNA detection [9], drug discovery and high-throughput screening [10] immunoassays [11], singlet oxygen generation [12] etc. to name but just a few.
Phosphorescent compounds, considered to be long-lived luminescent compounds, have the ability to store light and then slowly release such light energy. Phosphorescent compounds can move from the ground state (singlet state) to a metastable (triplet state) and slowly decay back to the ground state. Thus, a phosphorescent compound can emit light ever after the excitation source is removed. However, the intensity of the emission can be greatly reduced because of this slow release of energy.
A fluorescent compound glows under the excitation of an energy source and as soon as the energy source is removed, the compound stops emitting light. However, long-lived fluorescence molecules are different, in that, such compounds show a delayed fluorescence (α-S1-3S0) and phosphorescence (T1→S0) at room temperature along with traditional fluorescence (S1-*So). This type of long lived fluorescence i.e. delayed fluorescence arises due to different mechanisms [24]. The population of the excited singlet state (Si) by thermal activation of the triplet state (Ti) is responsible for E type delayed fluorescence, whereas a triplet-triplet annihilation process gives rise to P-type delayed fluorescence. The third type of delayed fluorescence is attributed to the recombination of radial ions and matrix trapped electrons produced by photoionization of the molecules. Because of its long decay time, delayed fluorescence has been widely used to investigate the rotational diffusion time of biological macromolecules in membranes and also used to characterized metal oxide surfaces [12].
However, because of the delayed emissions and lower intensity of such emissions, many important luminescent compounds are not used as high flux luminescent materials. As such, it would be advantageous to provide for systems and methods that can provide for shorter lived and brighter higher photon flux luminescent materials.